1. Field of the Invention
Embodiments of the invention relate to a liquid crystal display, and more particularly, to a horizontal electric field type liquid crystal display capable of improving a luminance and a contrast ratio.
2. Discussion of the Related Art
A liquid crystal display generally displays an image by controlling a light transmittance of liquid crystals using an electric field. The liquid crystal display may be roughly classified into a vertical electric field type liquid crystal display and a horizontal electric field type liquid crystal display depending on a direction of the electric field driving the liquid crystals. In the vertical electric field type liquid crystal display, a common electrode on an upper substrate and a pixel electrode on a lower substrate are positioned opposite each other, and liquid crystals of a twisted nematic(TN) mode are driven by a vertical electric field generated between the common electrode and the pixel electrode. The vertical electric field type liquid crystal display has an advantage of a large aperture ratio, but has a disadvantage of a narrow viewing angle. In the horizontal electric field type liquid crystal display, liquid crystals of an in-plane switching (IPS) mode are driven by a horizontal electric field generated between a common electrode and a pixel electrode that are positioned parallel to each other on a lower substrate. The horizontal electric field type liquid crystal display has an advantage of a wide viewing angle.
FIG. 1 is a plane view showing a pixel area of a related art horizontal electric field type liquid crystal display. FIG. 2 is a cross-sectional view taken along line I-I′ of FIG. 1. FIG. 3 illustrates a simulation result of a transmittance of FIG. 1. In FIG. 3, a black portion indicates an area that prevents light from being transmitted, and a white area indicates an area that permits light to be transmitted.
As shown in FIG. 1, the related art horizontal electric field type liquid crystal display includes a thin film transistor (TFT) at a crossing of a data line DL and a gate line GL, a pixel electrode 10 connected to the TFT, a common electrode pattern 20 to which a common voltage is supplied, and a pixel common electrode 30 that is connected to the common electrode pattern 20 and is opposite to the pixel electrode 10. In the related art horizontal electric field type liquid crystal display, a pixel area P is divided into two domains, namely, a first domain D1 and a second domain D2, so as to widen a viewing angle.
The TFT includes a gate electrode connected to the gate line GL, a source electrode connected to the data line DL, and a drain electrode connected to the pixel electrode 10 through a first passivation hole PH1. The TFT is turned on, and thus a data voltage from the data line DL is supplied to the pixel electrode 10. The pixel electrode 10 includes a vertical portion 10a formed parallel to the data line DL at one side of the pixel area P and a slant portion 10b extending from the vertical portion 10a to form a predetermined slant angle with the gate line GL. The pixel common electrode 30 includes a vertical portion 30a formed parallel to the data line DL at the other side opposite the one side of the pixel area P and a slant portion 30b extending from the vertical portion 30a to form a predetermined slant angle with the gate line GL. The slant portion 10b of the pixel electrode 10 includes a plurality of finger portions having an up-down symmetric structure based on a domain boundary between the first domain D1 and the second domain D2, and the slant portion 30b of the pixel common electrode 30 includes a plurality of finger portions having an up-down symmetric structure based on the domain boundary. The finger portions of the slant portion 10b and the finger portions of the slant portion 30b are positioned parallel to and opposite one another on the same domain to be spaced apart from one another at a predetermined distance. The common electrode pattern 20 includes a first pattern 20a and a second pattern 20b. The first pattern 20a partially overlaps the pixel electrode 10 to form a storage capacitor Cst. The second pattern 20b partially overlaps the pixel common electrode 30 around the domain boundary and is electrically connected to the pixel common electrode 30 through a second passivation hole PH2. Liquid crystals positioned corresponding to the first domain D1 and the second domain D2 are oriented symmetric to each other based on the domain boundary and are driven by a voltage difference between the data voltage applied to the pixel electrode 10 and the common voltage applied to the pixel common electrode 30.
The related art horizontal electric field type liquid crystal display has the following problems.
First, because the second passivation hole PH2 is formed over a display area around the domain boundary, a critical width W1 of the pixel common electrode 30 formed around the domain boundary has to increase so as to secure an overlay margin between the second passivation hole PH2 and the pixel common electrode 30. As shown in FIG. 3, because light is prevented from being transmitted through the pixel electrode 10 and the pixel common electrode 30, an increase in the critical width W1 of the pixel common electrode 30 causes a reduction in a transmittance and a luminance.
Second, as shown in FIG. 2, the pixel common electrode 30 has a step portion along edges A and B of the second pattern 20b of the common electrode pattern 20 because of a thickness of the common electrode pattern 20 contacting the pixel common electrode 30 through the second passivation hole PH2. The liquid crystals positioned corresponding to the first domain D1 are out of alignment because of the step portion of the pixel common electrode 30. Hence, light leakage is caused. As a result, a black luminance increases, and an entire contrast ratio decreases.
Third, a distance between an uppermost slant portion 10b of the pixel electrode 10 and an uppermost slant portion 30b of the pixel common electrode 30 has to be equal to or greater than a critical distance L2, so as to prevent a short circuit. In a process for preventing the short circuit, an opening having a predetermined distance L1 is formed between the pixel electrode 10 and the pixel common electrode 30 that are opposite to a previous gate line. Because the predetermined distance L1 is determined depending on the critical distance L2, it is difficult to set the predetermined distance L1 to a value less than a distance L3 between the finger portions of each of the slant portions 10b and 30b. 
As above, in the related art horizontal electric field type liquid crystal display, because the opening has the wide distance L1, an electric field is greatly generated between the pixel electrode 10 and the previous gate line, and between the pixel common electrode 30 and the previous gate line, and the liquid crystals are driven in an area affected by the electric field to generate the light leakage. Accordingly, in the related art horizontal electric field type liquid crystal display, the size of a black matrix pattern BM entirely covering the area affected by the electric field has to increase so as to prevent the light leakage. However, an increase in the size of the black matrix pattern BM reduces the entire luminance of the related art horizontal electric field type liquid crystal display.